All we know about the universe is in what is called "the observable universe," because there's much beyond what even the most powerful modern telescopes and data-retrieval systems can detect. And something out there is exerting power over other things we can observe. We can't see it, and we don't know what it is, because it's so far away.
In recent years, scientists began noticing something a little bit off about the structure of the universe. By analyzing the light from distant galaxies, they were able to tell the relative speed and direction in which these objects were moving. The strange thing is that, rather than flying apart like most things in the universe, some of these distant galactic clusters appear to be caught up in a sort of current, speeding at unimaginable velocities (about two million miles per hour) along a specific path. Scientists have coined this phenomenon “dark flow” because, honestly, they really don’t know what’s causing it.
For gravity to be acting on these clusters the way that it seems to be, there would have to be something massive waiting at the end of the path. By massive, we mean something potentially much bigger than anything that we’ve ever observed in the known universe; something big enough to absolutely dwarf the galactic clusters being sucked towards it like dust to a vacuum cleaner.
Read more on what we know and what we don't, at Environmental Graffiti. Link